Magnetic random access memories (MRAMs) employ magnetic multilayer films as storage elements. When in use, an MRAM cell stores information as digital bits, which in turn depend on the alternative states of magnetization of thin magnetic multilayer films forming each memory cell. As such, the MRAM cell has two stable magnetic configurations, high resistance representing a logic state 0 and low resistance representing a logic state 1, or vice versa.
A typical multilayer-film MRAM includes a number of bit or digit lines intersected by a number of word lines. At each intersection, a film of a magnetically coercive material is interposed between the corresponding bit line and digit line. Thus, this magnetic material and the multilayer films from the digit lines form a magnetic memory cell which stores a bit of information.
The basic memory clement of an MRAM is a patterned structure of a multilayer material, which is typically composed of a stack of different materials, such as copper (Cu), tantalum (Ta), permalloy (NiFe) or aluminum oxide (Al2O3), among others. The stack may contain as many as ten different overlapping material layers and the layer sequence may repeat up to ten times. Fabrication of such stacks requires deposition of the thin magnetic materials layer by layer, according to a predefined order.
FIG. 1 shows an exemplary conventional MRAM structure including MRAM stacks 22 which have three respective associated bit or digit lines 18. The digit lines 18, typically formed of copper (Cu), are first formed in an insulating layer 16 formed over underlayers 14 of an integrated circuit (IC) substrate 10. Underlayers 14 may include, for example, portions of integrated circuitry, such as CMOS circuitry. A pinned layer 20, typically formed of ferromagnetic materials, is provided over each digit line 18. A pinned layer is called “pinned” because its magnetization direction does not rotate in the presence of applied magnetic fields.
Conventional digit lines and pinned layers, such as the digit lines 18 and the pinned layers 20 of FIG. 1, are typically formed by a damascene process. Although damascene processes are preferred for copper interconnects, in the MRAM cell context the damascene process poses a drawback, in that there is an overlay of the pinned layer 20 with respect to the associated digit line 18, which occurs primarily as a result of photoresist misalignment. On FIG. 1, this overlay is illustrated by an overlay distance D, on each side of the digit line 18. Because of technical and processing limitations, conventional damascene processing is also unable to obtain long digit lines and their respective pinned layers.
Another drawback of using a conventional damascene process to produce the digit lines 18 of an MRAM is the inability of the process to achieve a minimal space or minimum critical dimension CD (FIG. 1) between two adjacent digit lines and, consequently, between two adjacent memory cells. Current values of the minimal space or critical dimension are in the range of 0.20 μm. However, with increased packing density of MRAM cells, the minimal space must decrease to values less than or equal to 0.1 μm, or even less than or equal to 0.05 μm, and current damascene processing does not afford these values with current 248 nm lithography.
Accordingly, there is a need for an improved method for fabricating MRAM structures, such as pinned layers and digit lines, which are minimally spaced from each other, as well as a method for decreasing the critical dimension between two adjacent MRAM structures formed on an integrated circuit substrate.